Experimental Study of Adsorptive Interactions of Polar and Nonpolar Adsorbates in the Zeolitic Imidazolate Framework ZIF-68 via Pulse Gas Chromatography

Low coverage adsorptive properties of linear and branched alkanes, 1-alkenes, cycloalkanes, aromatics, and small polar adsorbates on the Zeolitic Imidazolate Framework ZIF-68 were explored by pulse gas chromatographic experiments at temperatures between 413 and 513 K. ZIF-68 has a complex structure with amphiphilic properties due to the presence of two different imidazole linkers (2-nitroimidazolate and benzimidazolate), resulting in small polar and large nonpolar channels. Shape selective properties are absent for linear and branched alkanes (C-5-C-12), which is not surprising given the occurrence of large channels in the structure. A specific mechanism for the adsorption of polar adsorbates is proposed via strong interactions with the nitro groups, pointing inward in the small channel. Further, n-alkanes, 1-alkenes, cycloalkanes, and aromatics were used as probe molecules to determine whether or not specific interactions were involved in the framework for nonpolar adsorbates. These experimentally obtained adsorption energy values were linked to a simple model, which relates the adsorption enthalpy to the degree of branching and to the contribution of nonspecific van der Waals (number of hydrogen atoms) and electrostatic (number of double bonds) interactions.